1. Field of the Invention
The present invention relates generally to the field of semiconductor fabrication and, more particularly, to a method and related operation system for immersion lithography.
2. Description of the Prior Art
Photolithographic systems have been a mainstay of semiconductor device patterning for decades. In photolithographic systems, as known in the art, light is projected onto a photoresist for the purpose of patterning an electronic device on a semiconductor substrate or wafer.
The resolution (r0) of a photolithographic system having a given lithographic constant k1, is described by the equationr0k1λ/NA  (1)
where λ is the operational wavelength, and the numerical aperture (NA) is given by the equationNA=n sin θ0  (2)
The angle θ0 is the angular semi-aperture of the system, and n is the index of the material filling the space between the system and the substrate to be patterned.
Conventional methods of resolution improvement have led to three trends in the photolithographic technology: (1) reduction in wavelength λ from mercury g-line (436 nm) to the 193 nm excimer laser, and further to 157 nm and the still developing extreme-ultraviolet (EUV) wavelengths; (2) implementation of resolution enhancement techniques such as phase-shifting masks, and off-axis illumination have led to a reduction in the lithographic constant k1 from 0.6 to values approaching 0.4; and (3) increases in the numerical aperture (NA) via improvements in optical designs, manufacturing techniques, and metrology. Such improvements have led to increases in NA from approximately 0.35 to greater than 0.7, with 0.8 expected in the next few years. However, as can be seen in Equation (2), for free-space optical systems (i.e., n=1), there is a theoretical limit bounding NA to values of one or less.
Immersion lithography provides another possibility for increasing the NA of an optical lithographic system. Immersion lithography is a technology in which lithographic exposure of a resist coated wafer is performed with immersion fluid such as purified water introduced between the projection lens of a stepper and the wafer. The light source of the leading-edge stepper currently used in production lines is the ArF 193 nm excimer laser, and its resolution is approximately 90 nm. 65 nm is said to be the limit even for a system for research and development. The idea behind immersion lithography is to use the same ArF light source and yet realize a semiconductor process technology that achieves a resolution higher than 65 nm.
Because immersion lithography utilizes immersion fluid such as purified water to increase the NA of an optical lithographic system, the exposed wafers are subjected to thermal treatment by post exposure baking (PEB) to evaporate the fluid on the photoresist so as to avoid watermarks. In addition, according to the result of experiments, the length of wires on the wafers is related to the length of the duration when the wafers are waiting for baking after exposure. In general, the longer the duration is, the narrower the length of wires on the wafers is. Therefore, if the duration when the wafers are waiting for baking after exposure is too long, the circuits on the wafers could be broken down.